The present invention relates to low speed high torque (LSHT) gerotor motors, and more particularly to such motors which are associated with a gear reduction device, to further reduce the output speed of the motor and further increase the output torque of the motor.
Gerotor motors of the LSHT type are now quite well known in the art, and have been a substantial commercial success. Typically, a LSHT gerotor motor includes a gerotor displacement mechanism, valving for directing fluid from an inlet port to the expanding volume chambers of the gerotor, and for porting return fluid from the contracting volume chambers of the gerotor to the outlet port. Normally, the internally-toothed member of the gerotor is fixed to the motor housing, and the externally-toothed member orbits and rotates therein, in response to the introduction of pressurized fluid. The rotational component of the orbital and rotational movement of the externally-toothed member is transmitted to a rotating output shaft, typically by means of a universal connection, such as a shaft having crowned splines at each end.
A conventional LSHT gerotor motor made and sold commercially by the assignee of the present invention in which the gerotor element has a displacement of about 8.0 cubic inches per revolution would generate an output torque of about 2,100 inch pounds, with the output shaft rotating about 550 rpm, when fluid is communicated to the motor at a pressure differential of 2,000 psi and a flow rate of about 20 gpm.
In many applications for LSHT motors, such as wheel drives and winch drives, it is desirable to provide a driving force having an even higher torque, and an even lower speed. As long as LSHT gerotor motors have been in use, it has been common practice in the art to combine the motor with some sort of external, add-on gear reduction device for applications requiring higher torques and lower speeds. However, the addition of an external gear reducer to the gerotor motor is undesirable because it substantially increases the overall size, weight, and cost of the drive package. Thus, for many years there has been a need for a LSHT gerotor motor-gear reducer assembly not having the disadvantages of those in commercial use, but as of the filing of the present application, very low speed, very high torque drive packages almost always still consist of a gerotor motor and a gear reducer, purchased as separate units from separate suppliers.
U.S. Pat. No. 2,240,874 discloses a gerotor motor which utilizes a compound epicyclic gear train to provide a built-in gear reduction capability. The device shown in the '874 patent includes two gerotor elements, with the ring of the first gerotor being fixed, the stars of the first gerotor and the second gerotor being connected for orbital and rotational movement together, and the ring of the second gerotor element being connected to the output shaft. The ring and star of the first gerotor have a different number of teeth or lobes than the ring and star, respectively, of the second gerotor. In the '874 patent, in the first gerotor, the ring had 15 teeth and the star had 14 teeth, and in the second gerotor the ring had 14 teeth and the star had 13 teeth. Because of this difference in the number of teeth, as described in greater detail in the '874 patent, the result was a reduction ratio of 14:1, i.e., the output shaft generating a torque about 14 times that generated by the rotation of the stars, and rotating at a rate about 1/14 that of the stars.
Despite the teachings of U.S. Pat. No. 2,240,874 regarding the use of two gerotors to provide a built-in gear reduction, there has apparently never been any widespread commercialization of a device embodying the concepts taught therein, and the need for a LSHT motor-gear reducer package has continued to exist up until the time of the present invention, especially in the field of mobile hydraulics.
It should be noted that in the device of U.S. Pat. No. 2,240,874, pressurized fluid is communicated from the inlet port to the expanding volume chambers of both of the gerotor elements, thus requiring substantially more complicated and expensive valving than is commonly required in commercial LSHT motors. In addition, because fluid is communicated to and from the gerotors through an eccentric portion of a central shaft, the rings and stars of the gerotors must be quite large to accommodate this shaft and all the valving ports and passages, thus making the gerotor elements prohibitively expensive. Finally, the particular configuration used in the device of the '874 patent necessitated the use of seven separate bearing sets which also would be prohibitively expensive in a commercial device.